[AMRadio] retro 75


k1swl at earthlink.net k1swl at earthlink.net
Sat Apr 10 08:07:11 EDT 2010


Brett-

Glad you found a solution. So far, I haven't heard from anyone else
experiencing this issue.

I set the audio volume on the Retro-75 up for 'room-filling' levels. The
ALC control engages only at high signal levels and keeps the LM386 audio
final from clipping.  With the 3" speaker I'm using, I can fill the entire
house with audio before this happens. It's admittedly not much of an AGC
function per se. 

It wouldn't be too difficult to improve on the receive AGC action, but it
would take more complexity. In particular, an additional audio amp stage
would provide the levels needed for a lower AGC threshold. The AF gain
control would also need to be relocated outside the control loop.
Otherwise, the gain control does nothing once the threshold has been
reached. I solved this on the old WM-75 with a 100-ohm pot/rheostat right
at the audio output.

73- Dave Benson, K1SWL 


> [Original Message]
> From: Brett Gazdzinski <Brett.Gazdzinski at verizon.net>
> To: Discussion of AM Radio in the Amateur Service
<amradio at mailman.qth.net>
> Date: 4/9/2010 10:41:08 PM
> Subject: [AMRadio] retro 75
>
> I did some signal tracing tonight, and found the 1 meg resistor off the 
> detector seems to be the cause of my excessive rx audio distortion.
> Not sure if the fet detector (Q3) is bad or not, but with a 100k in place
of 
> the 1 meg, the audio cleans up nicely. I still have plenty of gain.
>
> I am not sure if anyone else is having distortion on the rx audio or not.
>
> With the resistor changed, the receiver sounds very good, but the lack of 
> any agc makes the levels all over the place....
>
> Brett
>
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